1. Field of the Invention
The present invention relates generally to devices which employ deep trenches, possibly for isolation, and more specifically to a structure to provide junction breakdown stability for deep trench devices.
2. Description of the Prior Art
Deep trench structures are attractive because of the improved density and performance they provide when used in high speed junction devices. They may be used in both isolated and non-isolated discrete devices depending on circuit requirements. Typical devices which employ isolated and non-isolated deep trench structures include transistors and diodes as well as a host of other junction devices.
Deep trench structures form plane junctions which have higher junction breakdown voltages than do planar structures. However, they have the same junction breakdown voltage instability problem that planar junction structures possess. This instability, often referred to as "walkout", results when oxide-passivated junctions are subjected to conditions which cause avalanche breakdown. This phenomenon occurs in both planar and deep trench structures and is time dependent; the breakdown voltage increases with the amount of avalanche current present and the time current flows through the junction.
Junction breakdown voltage instabilities are caused by surface effects. In deep trench structures, the primary, or metallurgical, junction terminates on the surface. Oxide passivated surfaces cause junction breakdown voltage instabilities because they trap charges in the sidewall dielectric (in this case, oxide). The increase in breakdown voltage is attributed to the injection of charges into the dielectric and a portion of the charges subsequently becoming trapped in the oxide.
As a result of charges being trapped in the oxide, the electric field decreases and the junction breakdown voltage increases to compensate for the trapped positive charge. High fields exist at the metallurgical junction where the junction breakdown occurs. These fields generate enough energy to inject charges into the oxide where some of the charges are trapped. This affects surface fields, which in turn destabilizes junction breakdown voltage.